Method of enhancing physical properties of aluminum base alloys containing zinc and magnesium



Patented May 9, 1950 UNITED STATEg FElCE Marmaduke A. Hobbs, South Bend, 1nd,, assignor to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Application June 8, 1946,

Serial No. 675,329

The present invention relates to a method of improving the strength and formability of wrought products made from aluminum base alloys containing substantial amounts of magnesium, zinc, and copper as the major alloying components aside from aluminum.

The alloys to which the present invention relates are of the class of high strength wrought alu minum base alloys requiring solution heat treatment, quenching, and artificial aging to develop their maximum mechanical properties. The behavior of the aluminum base alloys containing a substantial quantity of zinc, and lesser amounts of copper, and magnesium as the principal alloy components apart from aluminum, is different in many respects from that of other high strength aluminum base alloys, especially that of the socalled Duralumin type. Among the'important differences is the dissimilarity in behavior of the hardening constituents of the foregoing aluminummagnesium-zinc-copper type alloys and those of the aluminum-copper-magnesium type. For example, the former possess a much higher solid solubility than the latter at elevated temperatures. The age hardening at room temperature of the aluminum-zinc-magnesium-copper type alloys proceeds at a somewhat slower rate than that of the Duralumin type alloys and continues indefinitely, though at a decreasing rate. In contrast, the spontaneous aging of some Duralumin type alloys is comparatively rapid and is virtually complete at the expiration of four days after quenching. On the other hand, the age hardening at elevated temperatures of this type of aluminum-zinc-magnesium-copper alloys proceeds at a much faster rate than that of the Duralumin type alloys, and to obtain maximum tensile properties, the former alloys must be artificially aged at appreciably lower temperatures than the latter alloys. The alloys wherein these difierences and others enumerated below are particularly evident are those containing from 3 to zinc, 0.75 to 4% magnesium, and 0.05 to 3% copper, the zinc always being in excess of either the magnesium or copper components.

Another peculiarity of articles made from the aluminum-zipc-magnesiuin-copper type of alloy referred to herein is their response to severe forming operations after being artificially aged at a constant temperature. In such severe forming operations as dimpling of sheet preparatory to the insertion of countersunk rivets, the artificially aged aluminum-zinc-magnesium-copper alloy sheet is inferior to that of the aged aluminum copper-magnesium type alloy. It is this property of formability with which this inven- 9 Claims. (Cl. 14812.7)

tion is particularly concerned, especially where a high strength is also desired. Formability, as this term is generally understood, refers to the ability of a body toundergo non-uniform deformation in shaping operations without rupture or other failure. Dimpling is illustrative of a severe forming operation and can serve as a measure of relative formability between sheets of different materials. The elongation values of the type of alloy herein described which are obtained in the usual tensile tests are not, however, reliable indication of relative formability. i

Still another unusual characteristic of th solution heat treated and artificially aged articles of zinc-containing alloys referred to herein, and more fully described below, which I have dis-' covered, is that their formability does not d1- minish when they are cold worked between solution heat treatment and artificial aging even though their tensile and yield strengths are increased. In the case of other types of alloys similarly heat treated and cold worked the tensile and yield strengths are increased but the formability is decreased. In referring to yield strength it is to be understood that this means the strength as determined in tension and not in compression.

An object of the present invention is to provide a method for improving the formability of artificially aged wrought articles made from aluminum base alloys containing substantial amounts of zinc, magnesium, and copper wherein the quantity of zinc exceeds that of the magnesium or copper components. A further object is to provide a method of simultaneously improving the formability, and the tensile and yield strengths of artificially aged wrought articles made from aluminum base alloys containing a substantial amount of zinc and lesser amounts of magnesium and copper. A specific object of the invention is to provide a method of improving the dimpling capacity and the tensile and the yield strengths of artificially aged sheet products made from the type of aluminum-Zinc-magne sium-copper alloys described herein.

My invention is predicated on the discovery that the iormability, especially the capacity to undergo severe forming operations of solution heat treated and artificially aged wrought articles made from aluminum base alloys containing a substantial amount of zinc and lesser quantities of magnesium and copper can be improved and,

'atthe-same time, an increase in the tensile and yield strengths obtained by cold working the material a predetermined amount in the interval betweenquenching and artificial aging. The complete procedure for improving the formability and the strengths consists in solution heat treating the material for a time and at a temperature sufficient to obtain substantially complete solution of the soluble alloying constituents at the temperature employed. For alloys of this type, the solution heat treating temperature is preferably within a range of 860 to 940 F. and the period of treatment within a range of about minutes to one hour, the length of time required to obtain substantially complete solution of the soluble constituents in any instance depending upon the mass of the material being treated and upon the thickness of the sections comprising the article. When the articles have been solution heat treated for a sufficient time, they are removed from the heat treating furnace or bath and rapidly quenched to approximately room temperature. A rapid or drastic quench such as in cold water, is necessary if maximum strengths are to be obtained.

The cold working may be accomplished by one of a number of the common working operations such as rolling, stretching, or pressing. For convenience the amount of cold work will be expressed throughout the specification and in the appended claims in per cent of cold work without any special emphasis on the cold working operation involved. The per cent of cold work as used herein refers to that amount which corresponds to an equal numerical percentage reduction in thickness produced by cold rolling. For example, if a sheet is reduced 10 per cent in thickness by cold rolling, it would be considered to have received 10 per cent cold work and its strength and hardness would be increased a certain amount, and conversely, if the sheet developed substantially the same properties after cold working by other means than rolling, the sheet would be regarded as having been cold worked 10 per cent. Hence, in other working operations than rolling where the relationship of the reduction in thickness to the amount of cold work produced thereby may not be in the same ratio as that found in cold rolling, the amount of cold work is, nevertheless, considered to be the same as that obtained by cold rolling if the properties obtained by the cold working ar substantially equal to those produced by a given reduction in thickness.

The minimum per cent of cold work which is required to obtain an improvement in the strengths of the material is equal to a 4 per cent reduction in thickness by rolling, and, as mentioned above, this is considered to be 4 per cent cold work. Cold working in amounts less than about 4 per cent either fails to produce an increase in the tensile and yield strength or causes a slight decrease in both of these strengths although it may produce a slight improvement in the formability of the material. The strengths and formability progressively increase as the amount of work hardening is increased above about 4 per cent to approximately 35 per cent. It has been found, however, that for most commercial purposes cold work in amounts of less than about 20 per cent but more than 4 per cent provides a sufficient improvement in strength and formability of the material. As the amount of cold work increases, less artificial aging is required to produce the maximum strength which is possible with the result that it becomes in creasingly easy to overage the material and thereby obtain less than the maximum tensile properties.

The material may be cold worked at any time after quenching and before aging; i. e., the length of the interval between quenching and the cold working operation and the length of the interval between the cold working operation and the artificial aging treatment do not have any substantial effect on the strengths and formability of the final product in the artificially aged condition.

In the practice of my invention, the cold worked material is artificially aged at a temperature between about and 300 F. for a period of time sufiiciently long to attain maximum strength. Th length of time depends to a great extent upon the aging temperature employed and upon the amount of cold work given the material in the interval between quenching and artificial aging as mentioned above. A total aging time of between 6 and 36 hours is generally adequate to develop maximum strengths under most conditions. In aging, the material may be held at a constant temperature throughout the entire aging procedure or the temperature may be varied to suit the individual requirements of the material or of the plant fabricating conditions. That is, the aging may be effected by several separate treatments instead of one. The aging may be started by heating the article to a predetermined temperatur and holding it at that temperature for a period of time. The article is then heated to higher temperature and so on until the desired aging is completed. For example, sheet may be aged first at a relatively low artificial aging temperature and thereafter at a somewhat higher temperature to complete the aging, but the total time should lie between about 6 and 36 hours. The temperatures and times employed in the difierent aging practices can be varied to favor the development of certain properties or to conserve time in obtaining the desired properties. The artificial aging treatment which has been found to be satisfactory under many conditions is 16 to 28 hours at 250 F. As previously indicated, the actual length of time at 250 F. required to obtain maximum properties is determined by the amount of cold work given the material prior to the artificial aging treatment. Artific al aging temperatures above about 300 F. generally fail to develop the highest strengths obtainable in the cold worked material; furthermore, the aging progresses so rapidly that there is considerable danger of over-aging the material and thereby cause a decrease in strength. At temperatures below about 150 F., aging of the cold worked material progresses too slowly to be practicable in production.

As previously mentioned herein my invention is applicable to aluminum base alloys containing an appreciable amount of zinc, and lesser quantities of magnesium and copper, but more specifically those alloys consisting of about 3 to 10 per cent zinc, 0.75 to 4 per cent magnesium, 0.05 to 3 per cent copper, with or without 0.1 to 1.5 per cent manganese and with or without small amounts of one or more of the grain refining elements titanium, boron, zirconium, molybdenum, tungsten. cobalt, chromium, and vanadium, the balance being substantially all aluminum. The quantity of zinc should exceed that of magnesium or copper. A preferred range of composition is the following: 4 to 6 per cent zinc; 0.75 to 3.0 per cent magnesium, 0.1 to 2 per cent copper, 0.1 to 1 per cent manganese and at least one of the following elements: 0.02 to 0.25 per cent titanium, 0.005 to 0.1 per cent boron, 0.01 to 0.15 per cent zirconium, 0.02 to 0.25 per cent molybdenum, 0.02 to 0.2 per cent tungsten, 0.02 to 0.2 per cent cobalt, 0.05 to 0.5 per cent chromium, and 0.02 to 0.2 per cent vanadium, and a balance of substantially all aluminum. The expression a balance of substantially all aluminum does not exclude small amounts of the usual impurities, such as iron and silicon, found in commercially pure aluminum.

Illustrations of the effectiveness of the present method are to be found below in the results of tests made on .064" thick sheet, the nominal composition of which was as follows: 5.75 per cent zinc, 2.5 per cent magnesium, 1.6 per cent copper, 0.15 per cent manganese, 0.25 per cent chromium, 0.05 per cent titanium, 0.15 per cent silicon, 0.35 per cent iron, and balance aluminum. Two groups of samples were solution heat treated at 870 F. for 20 minutes and quenched in cold water. The samples of one group were cold rolled to a five per cent reduction in thickness and then artificially aged at 250 F. for 24 hours while samples of the other group were artificially aged by the same treatment without first receiving any cold work after quenching. The average results of the tensile, bend, and dimpling tests made on the several samples given the foregoing treatments are tabulated below:

TABLE I Efiect of cold work on strength and immobility Number of 90 Tensile Yield Percent ggfi Reverse Bends Strength, Strength, Elongation Dimming I .s.l. .S.l. ln-lnc es p p Parallel Normal SAMPLES NOT COLD WORKED AFTER QUENCHLN'G SAMPLES COLD WORKED 5% AFTER QUENCHING AND BEFORE ARTIFICIAL AGING The dimpling capacity as a measure of formability was determined in these tests by making dimples for four different sized rivets having 100 countersunk heads (type Army-Navy N426). The several dimple shes covered a range in which the largest size generally produced some failures whereas the smallest size generally produced only sound dimples. The dimpling tools were modified versions of those described in the National Advisory Committee on Aeronautics Technical Note No. 854. The dimples were radiographed to determine whether cracks had developed in the dimpling operation. The dimples having any cracks, however small, were marked failures. The reverse bends were made by bending the samples back and forth through a 90 angle over a mandrel having a 0.25 inch radius and noting the number of 90 bends made until failure occurred. A comparison of the results from the two groups of samples shows a measurable improvement in strength and an appreciable improvement in formability with virtually no change in elongation.

In another test to determine tensile properties alone, several samples of sheet of the same alloy were solution heat treated at 870 F. for 20 minutes, quenched in cold water, and various samples were given reductions in thickness by cold rolling 2, 4, 8, and 16 per cent and were then aged at 250 for 24 hours. One set of the samples was artificially aged after quenching without first be- 6 ing 'cold worked. The average results of the tensile tests made on these samples are tabulated below:

TABLE II Efiect of varying reductions on strength These results illustrate the increase in strengths which results from cold working the material about 4 per cent or more, as well as the decrease which results from cold working the sheet in an amount less than about 4 per cent.

In still another test, samples of sheet of the same alloy were solution heat treated and quenched as those in the previous tests, and then various samples were given reductions in thickness of 2 and 8 per cent before being artificially aged. These cold worked samples, together with a group that was not cold rolled, were first aged for '8 hours at 210 F. and then for 12 hours at 275 F. The results of dimpling tests of the kind described above made on these three groups of samples are given below.

TABLE III Eflect of cold work on formaln'lz'ty These results clearly show the improvement which can be obtained in the dimpling capacity or formability as a result of cold working the material in the interval between quenching and artificial aging.

I claim:

1. A method of increasing the tensile strength, yield strength, and formability of artificially aged wrought articles made from aluminum base alloys consisting essentially of 3 to 10 per cent zinc, 0.75 to 4 per cent magnesium, 0.05 to 3 per cent copper, and the balance aluminum, the amount of zinc exceeding that of either magnesium or copper, said method comprising solution heat treating said articles for a time and at a temperature suificient to obtain substantially complete solution of the soluble alloying constituents at that temperature, quenching, cold working the said articles between about 4 and 35 per cent, and thereafter artificially aging them between and 300 F. for a time between about 6 and 36 hours.

2. A method of increasing the tensile strength, yield strength, and formability of artificially aged wrought articles made from aluminum base alloys consisting essentially of 3 to 10 per cent zinc, 0.75 to 4 per cent magnesium, 0.05 to 3 per cent copper, and the balance aluminum, said method comprising solution heat treating said articles at a temperature between 860 and 940 F.

for about ten minutes to one hour, rapidly quenching them, cold working the articles between about 4 and 35 per cent and thereafter artificially aging them between 150 and 300 F. for a total time between about 6 and 36 hours.

3. 'A method of increasing the tensile strength, yield strength, and formability of artificially aged wrought articles made from aluminum base alloys consisting essentially of 3 to per cent zinc, 0.75 to 4 per cent magnesium, 0.05 to 3 per cent copper, and the balance aluminum, the amount of zinc exceeding that of either magnesium or copper, said method comprising solution heat treating said articles for jtime and at a temperature sufficient to obtain substantially complete solution of the soluble alloying constituents at that temperature, quenching, cold working the said articles between about 4 and 35 per cent, and thereafter artificially aging them within the temperature range of 150 to 300 F. by a plurality of separate treatments at successively higher temperatures.

4. A method of increasing the tensile strength, yield strength, and formability of artificially aged wrought articles made from aluminum base alloys composed of from 4 to 6 per cent zinc, 0.75 to 3.0 per cent magnesium, 0.1 to 2 per cent copper, 0.1 to 1 per cent manganese, and at least one of the group of grain refining elements consisting of 0.02 to 0.25 per cent titanium, 0.005 to 0.1 per cent boron, 0.01 to 0.15 per cent zirconium, 0.02 to 0.2 per cent cobalt, 0.02 to 0.25 per cent molybdenum, 0.02 to 0.2 per cent tungsten, 0.05 to 0.5 per cent chromium, 0.02 to 0.2 per cent vanadium, and the balance substantially all aluminum, said method comprising heat treating said articles at a temperature between 860 and 940 F. for about ten minutes to one hour, quenching them in cold water, cold working said articles between about 4 and per cent and thereafter artificially aging them between 150 and 300 for a total time between about 6 and 36 hours.

5. A method of increasing the tensile strength, yield strength, and formability of artificially aged wrought articles made from aluminum base alloys composed of from 4 to 6 per cent zinc, 0.75 to 3.0 per cent magnesium, 0.1 to 2 per cent copper, 0.1 to 1 per cent manganese, and at least one of the group of grain refining elements consisting of 0.02 to 0.25 per cent titanium, 0.005 to 0.1 per cent boron, 0.01 to 0.15 per cent zirconium, 0.02 to 0.25 per cent molybdenum, 0.02 to 0.2 per cent tungsten, 0.02 to 0.2 per cent cobalt, 0.05 to 0.5

. per cent chromium, 0.02 to 0.2 per cent vanadium,

and. the balance substantially all aluminum, said methcd comprising heat treating said articles at a temperature between 860 and 940 F. for about 10 minutes to 1 hour, quenching them, cold working said articles between about 4 and 35 per cent, and thereafter artificially aging them in two steps within the temperature range of 150 to 300 F. comprising effecting a portion of the aging at a predetermined temperature and continuing the aging at a higher temperature, the total time for said aging being between about 6 and 36 hours.

6. A method of increasing the tensile strength, yield strength, and formability of artificially aged aluminum base alloy sheet composed of from 4 to 6 per cent zinc, 0.75 to 3.0 per cent magnesium, 0.1 to 2 per cent copper, 0.1 to 1 per cent manganese, and at least one of the. group of grain refining elements consisting of 0.02 to 0.25. per cent titanium, 0.005 to 0.1 per cent boron, 0.01 to 0.15 per cent zirconium, 0.02 to 0.25 per cent molybdenum, 0.02 to 0.2 per cent tungsten, 0.02 to 0.2 per cent cobalt, 0.05 to 0.5 per cent chromium, 0.02 to 0.2 per cent vanadium, and the balance substantially all aluminum, said method comprising sclution heat treating said alloy sheet at a temperature between 860 and 940 for 10 minutes to 1 hour, quenching the sheet in cold water, cold rolling the sheet to between 4 and 20 per cent reduction in thickness and thereafter artificially aging the sheet between and 300 F. for a total time between about 6 and. 36 hours.

7. A method of increasing the tensile strength, yield strength, and formability of artificially aged wrought articles made from aluminum base alloys consistin essentially of 3 to 10 per cent zinc, 0.75 to 4 per cent magnesium, 0.05 to 3 per cent copper, and the balance aluminum, said method comprising solution heat treating said articles for a time and at a temperature suflicient to obtain substantially complete solution of the soluble alloying constituents at that temperature, quenching, cold rollin the said articles with a reduction in thickness of 4 to 35 per cent, and thereafter artificially aging them within the temperature range of 150 to 300 F. for a period of to 36 hours.

3. In the process of forming articles from a wrought aluminum base alloy product consisting essentially of 3 to 10 per cent zinc, 0.75 to 4 per cent magnesium, 0.05 to 3 per cent copper and the balance aluminum, the steps comprising solution heat treating the article for a time and at a temperature suificient to obtain substantially complete solution of the soluble alloying constituents at that temperature, quenching, cold working the article 4 to 35 per cent, artificially aging within the temperature range of 150 to 300 F. for a total period of 6 to 36 hours and thereafter forming the article.

9. In the process of forming articles from sheet of an aluminum base alloy consisting essentially of 3 to 10 per cent zinc, 0.75 to 4 per cent magnesium, 0.05 to 3 per cent copper and the balance aluminum, the steps comprising solution heat treating the sheet at a temperature between 860 REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,704,253 Hybinette Mar. 5, 1929 2,240,940 Nock, Jr. May 6, 1941 OTHER REFERENCES Age Hardening of Metals, by American So.- ciety for Metals, Cleveland, Ohio, 1940, p. 321. 

8. IN THE PROCESS OF FORMING ARTICLES FROM A WROUGHT ALUMINUM BASE ALLOY PRODUCT CONSISTING ESSENTIALLY OF 3 TO 10 PER CENT ZINC, 0.75 TO 4 PER CENT MAGNESIUM, 0.05 TO 3 PER CENT COPPER AND THE BALANCE ALUMINUM, THE STEPS COMPRISING SOLUTION HEAT TREATING THE ARTICLE FOR A TIME AND AT A TEMPERATURE SUFFICIENT TO OBTAIN SUBSTANTIALLY COMPLETE SOLUTION OF THE SOLUBLE ALLOYING CONSTITUENTS AT THAT TEMPERATURE, QUENCHING, COLD WORKING THE ARTICLE 4 TO 35 PER CENT, A, COLD WORKING THE ARTICLE 4 TO 35 PER CENT, ARTIFICIALLY AGING WITHIN THE TEMPERATURE RANGE OF 150* TO 300*F. FOR A TOTAL PERIOD OF 6 TO 36 HOURS AND THEREAFTER FORMING THE ARTICLE. 